1. Field of the Invention
The present invention relates to switch drive circuits.
2. Description of Related Art
Conventionally, a technology has been developed to suppress a surge voltage (instantaneous spike voltage) that occurs when a switching device such as an IGBT (insulated-gate bipolar transistor) turns off.
This technology will be described specifically with reference to FIGS. 6 to 8. Suppose that, as shown in FIG. 6, a gate driver 100 drives a gate of a switching device Q1 which is an IGBT.
In the configuration shown in FIG. 6, as shown in a timing chart in FIG. 7, in a case where, when the switching device Q1 turns off, a gate-emitter voltage Vge of the switching device Q1 is controlled so as to drop to the ground potential of the gate driver 100, the collector-emitter voltage Vce of the switching device Q1 may instantaneously rise up to a high voltage, causing a surge voltage. Here, the collector current Ice, which has been high with the switching device Q1 on, drops to zero. As described above, when the switching device Q1 turns off, a surge voltage may occur. In particular, when the collector current Ice is a large current, the surge voltage may exceed the withstand voltage of the switching device Q1, destroying the switching device Q1.
As a solution, conventionally, as shown in FIG. 8, when the switching device Q1 turns off, the gate-emitter voltage Vge of the switching device Q1 is controlled so as to drop initially to a voltage Vm higher than the threshold voltage Vt and then, after the lapse of a predetermined time, further to the ground potential of the gate driver 100. This helps suppress an instantaneous rise up to a high voltage in the collector-emitter voltage Vce of the switching device Q1, thus preventing a surge voltage from exceeding the withstand voltage of the switching device Q1.
One example of conventional technology related to the above description can be found in JP-A-2014-093836 and JP-A-2014-529239.
However, as will be understood from an overlap, indicated by hatching in FIG. 8, between the collector current Ice and the collector-emitter voltage Vce, a configuration that suppresses a rise due to a surge voltage when the switching device Q1 turns off suffers from a large switching loss. As a result, inconveniently, both in the control shown in FIG. 7 and in regular operation in which no surge voltage exceeding the withstand voltage of the switching device Q1 occurs when it turns off, a large switching loss occurs.